FFT

• Add option in Sensor to use a fast Fourier transform for the convolution of induced current and transfer function instead of doing the convolution in the time domain.
• For now use the FFT implementation from Numerical Recipes.
• TODO
  • Optimize the FFT implementation and/or use FFTW (through ROOT TVirtualFFT).
  • Cache the Fourier transform of the transfer function (instead of recomputing it at every call).
  • Avoid boundary effects due to periodicity.